Electroacoustic devices are employed, e.g., as microphones, loudspeakers, and telephone receivers and may be designed in a variety of ways. For example, so-called ring armature electroacoustic transducers comprise an annular permanent magnet, and an annular, magnetically permeable component known as a pole piece which complements the permanent magnet to a magnetic circuit having an air gap. An induction coil is in proximity of the permeable component and a diaphragm is physically attached to a magnetic element in the air gap. Such design is disclosed, e.g., in U.S. Pat. No. 2,506,624, "Electroacoustic Transducer", issued May 9, 1950, to R. E. Wirsching and also on p. 111 and p. 124 of the paper by E. E. Mott et al., "The Ring Armature Telephone Receiver", Bell System Technical Journal, Vol. 50, January 1951, pp. 110-140. The latter reference, on p. 112, further discloses three alternate transducer designs, two of which have axially symmetrical design similar to the ring armature transducer. Design of an electroacoustic transducer typically involves consideration of a variety of design parameters such as, e.g., choice of an appropriate combination of permanent magnet and permeable components. In this respect, certain alloy combinations have become established such as, in particular, combinations of Permalloy with either Remalloy or Alnico. The latter two alloys are mentioned e.g., in the book by R. J. Parker et al., Permanent Magnets and Their Application, Wiley, 1962.
Relevant with respect to the invention is a line of development which is concerned with Fe-Cr-Co alloys and their properties. Such alloys are disclosed in various publications and patents such as, e.g., U.S. Pat. No. 4,075,437, "Composition, Processing and Devices Including Magnetic Alloy", issued Feb. 21, 1978, to G. Y. Chin et al., U.S. Pat. No. 4,174,983, issued Nov. 20, 1979 to G. Y. Chin et al. U.S. patent application Ser. No. 924,138, filed July 13, 1978 (now abandoned in favor of continuation application Ser. No. 092,941), U.S. patent application Ser. No. 016,115, filed Feb. 28, 1979, and concurrently filed U.S. patent applications Ser. No. 069,277 and Ser. No. 069,278.
Relevant with respect to the invention is another line of development which is concerned with Fe-Al alloys and their properties. In particular, the paper D. Pavlovic et. al., "Study of Heat Treatments for Low Coercive Force 14 to 17 Percent Aluminum Iron Alloys", J. App. Phys., Supp. Vol. 31, No. 5, May 1960, pp. 231S-232S which is concerned with the influence of processing parameters on the coercive force of Fe-Al alloys containing 14-17 weight percent Al and, optionally, small ternary additions. A comparison of various magnetically soft alloys is made by E. Adams, "Recent Developments in Soft Magnetic Alloys", J. App. Phys., Supp. Vol. 33, No. 3, March 1962, pp. 1214-1220. Among alloys considered by Adams is a Si-Al-Fe alloy which is designated Sendust and which is also investigated in the paper by H. H. Helms, "Sendust Sheet-Processing Techniques and Magnetic Properties", J. App. Phys., Vol. 35, No. 3, March 1964, pp. 871-872. Fe-Al alloys containing 3 percent and 9 percent aluminum are considered, respectively, in the paper by K. Foster et al., "Magnetic Properties of Oriented 3 Percent Aluminum Iron Sheets", J. App. Phys., Vol. 34, No. 4 (Part 2), April 1963, pp. 1325-1326 and in the paper by D. Pavlovic et al., "Alloys with Low Remanence and Low Coercive Force in the Region of 9 Percent Aluminum Iron", J. App. Phys., Vol. 36, No. 3, pp. 1237-1239. Fe-Al alloys are also mentioned in the book by R. M. Bozorth, Ferromagnetism, Van Nostrand, 1951, pp. 210-220.